Sodium carbonate (Na2CO3), also called soda ash, is an important, high volume chemical produced in the United States and used in the manufacture of glass, chemicals, soaps and detergents, and aluminum, as well as in textile processing, petroleum refining and water treatment, among many other uses.
In the United States, almost all soda ash (sodium carbonate, Na2CO3) is obtained from subterranean deposits of naturally-occurring trona ore (as a primary source) and nahcolite ore (as a secondary source). The largest known trona ore deposits in the United States are located in the Green River Basin in southwestern Wyoming, mostly in Sweetwater County, Wyo., and are typically about 800 to 3000 feet below ground level. Nahcolite ore (naturally-occurring sodium bicarbonate, NaHCO3) deposits are located in the Piceance Creek Basin in northwest Colorado.
The subterranean deposits of trona ore consist primarily (80-95 wt. %) of sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O) and contain lesser amounts of sodium chloride (NaCl), sodium sulfate (Na2SO4), organic matter, and insolubles such as clay and shales.
Trona ore may be recovered from subterranean trona ore deposits, for further processing in surface operations into soda ash or other alkali products, by mechanical mining techniques or by various solution mining methods. The Green River trona ore deposits are presently being commercially mined both by mechanical mining and by solution mining processes. The Piceance Creek nahcolite ore deposits are being mined by solution mining techniques.
Soda ash production facilities recovering soda ash from trona ore deposits are operated by a number of different organizations in the Green River region of southwestern Wyoming, including FMC Corporation (the assignee of this patent application), Solvay Chemicals, Inc., Tata Chemicals North America and OCI Wyoming LP.
Mechanical mining, also called dry mining, is carried out underground in the subterranean alkali ore beds by mining crews using complex machinery and includes room-and-pillar and long wall mining methods. Mechanical mining methods are relatively costly due to the upfront cost of sinking mine shafts and continuing need for mining manpower and complex mining machinery. In addition, such mechanical mining methods leave unrecovered a significant fraction of the trona ore in the beds being dry mined, e.g., about 60% unrecovered in room-and-pillar mining and about 30% in longwall mining.
Solution mining is an alternative mining approach for recovering minerals from subterranean ore deposits. Solution mining is sometimes referred to as in situ recovery or in situ leaching. Solution mining can be utilized either as an alternative to or as a supplement to mechanical mining, for the economical recovery of subterranean mineral ore values, such as in the recovery of alkali values from trona ore as soda ash.
Solution mining utilizes conventional well drilling technology and involves injecting water or other aqueous-based mining solvent, via a drilled well hole, into a subterranean deposit of trona ore (or other soluble mineral ore); allowing the mining solvent to dissolve soluble ore; pumping the resulting mining solution (mine water) via a drilled well hole to the surface; and processing the mine water to recover dissolved ore values from the solution as solid products, in the form of sodium carbonate or other related sodium-based chemicals. Solution mining methods may also be employed for recovering alkali values from depleted ore deposits that have previously been mechanically mined and abandoned.
Numerous solution mining methods are disclosed in the patent literature for recovery of trona and nahcolite ores, using surface-initiated well drilling techniques to inject a variety of aqueous mining solvents to solubilize the subterranean ore deposit and subsequently recover an alkaline mining solution from the solution-mined ore deposit.
Exemplary solution mining processes for trona are disclosed in U.S. Pat. No. 2,388,009 of Pike issued Oct. 30, 1945; U.S. Pat. No. 3,050,290 of Caldwell et al. (FMC) issued Aug. 21, 1962; U.S. Pat. No. 3,119,655 of Frint et al. (FMC) issued Jan. 28, 1964; U.S. Pat. No. 3,184,287 of Gancy (FMC) on May 18, 1965; U.S. Pat. No. 4,264,104 of Helvenston et al. (PPG) issued Apr. 28, 1981; U.S. Pat. No. 5,043,149 of Frint et al. (FMC) issued Aug. 27, 1991; and U.S. Pat. No. 5,192,164 of Frint et al. (FMC) issued Mar. 9, 1993.
Examples of solution mining procedures applicable to nahcolite ore are described in U.S. Pat. No. 3,779,602 of Beard et al. (Shell Oil) issued Dec. 18, 1973; U.S. Pat. No. 4,815,790 of Rosar et al. (NaTec) issued Mar. 28, 1989; U.S. Pat. No. 6,699,447 of Nielsen et al. (American Soda) issued Mar. 2, 2004; and U.S. Patent Application Publication No. 2009/0200854 A1 of Vinegar (Shell Oil) published Aug. 13, 2009.
An alkali solution from solution mining of a NaHCO3-containing ore deposit such as trona or nahcolite typically contains dissolved sodium carbonate and sodium bicarbonate, as well as dissolved organic and inorganic impurities solubilized from the ore deposit. Alkali solutions containing Na2CO3 and NaHCO3 values may be obtained not only via solution mining of NaHCO3-containing subterranean ore deposits but also from surface alkali brine lakes or alkali waste ponds. The sodium carbonate values in such alkali solutions are normally recovered as soda ash by various crystallization processes, and the impurities present in the alkali solution are typically removed via a purge stream of crystallizer mother liquor, which is discarded.
Numerous processes have been described in the patent literature for treating aqueous alkali solutions obtained from solution mining or from surface lakes and ponds, to recover sodium carbonate and/or sodium bicarbonate from such alkali solutions. Many include a step of decomposing or neutralizing sodium bicarbonate in the aqueous alkali solution, a step that has the objective of increasing the sodium carbonate concentration and minimizing the sodium bicarbonate content, to facilitate crystallization of sodium carbonate, e.g., as sodium carbonate monohydrate, which can then be calcined to produce soda ash.
The following patents are exemplary of such prior art soda ash processes.
U.S. Pat. No. 4,869,882 of Dome et al. (General Chemical) issued Sep. 26, 1989 describes a process for recovering soda ash from waste or storage ponds associated with a soda ash manufacturing facility, via neutralization of the alkali waste water with lime to convert bicarbonate to carbonate, evaporation, and then crystallization of sodium carbonate decahydrate, which is recovered.
Surface processing operations for recovering soda ash from dry-mined trona ore and from alkali mining solutions obtained from trona solution mining are described in U.S. Pat. No. 5,262,134 of Frint et al. (FMC) issued Nov. 16, 1993.
Frint et al. '134 describes the recovery of sodium carbonate values from mining liquor obtained from solution mining of subterranean trona ore deposits, via sequential crystallizations of sodium sesquicarbonate and sodium carbonate decahydrate, the latter then being recrystallized as sodium carbonate monohydrate. The Frint '134 patent also contains descriptions of various prior art trona ore solution mining techniques, as well as descriptions of surface processing operations: the “sesquicarbonate” and “monohydrate” soda ash crystallization processes used for recovery of soda ash from dry-mined trona ore. Those disclosures of U.S. Pat. No. 5,262,134 are hereby incorporated by reference into the present specification.
Numerous soda ash recovery processes have been described in the patent literature for treating alkali solutions obtained from solution mining, and many include a step of decomposing sodium bicarbonate in the alkali solution, with the concurrent evolution of gaseous carbon dioxide, to covert the bicarbonate into sodium carbonate.
U.S. Pat. No. 5,283,054 of Copenhafer et al. (FMC) issued Feb. 1, 1994 describes a process for recovering sodium carbonate from aqueous mining solution obtained from solution mining of subterranean trona deposits. The process first converts sodium bicarbonate present in the aqueous mining solution to sodium carbonate, via evaporation and CO2 stripping, followed by neutralization with lime to decompose residual sodium bicarbonate in the evaporated solution. An intermediate product, sodium carbonate decahydrate, is crystallized from the NaHCO3-depleted solution and recovered, then redissolved and recrystallized as sodium carbonate monohydrate. The soda ash recovery process of the Copenhafer '054 patent is sometimes referred to as the Evaporation, Lime, Decahydrate, Monohydrate (ELDM) process.
Other soda ash recovery processes, analogous to the ELDM process, have been described in subsequent patents for recovery sodium carbonate values from alkali solutions.
U.S. Pat. No. 5,766,270 of Neuman et al. (Tg Soda Ash) issued Jun. 16, 1998 and U.S. Pat. No. 5,955,043 of Neuman et al. (Tg Soda Ash) issued Sep. 21, 1999 each describe processes for recovering sodium carbonate from dilute solution mining brines. In Neuman et al. '270, the sodium bicarbonate content of the mining brine is first lowered, via steam stripping, followed by crystallization of sodium carbonate decahydrate. In Neuman et al. '043, the bicarbonate content of the mining brine is first lowered, via neutralization with caustic soda or dilution, followed by crystallization of sodium carbonate decahydrate. Residual bicarbonate in the decahydrate mother liquor is removed via steam stripping.
Other patents and published patent applications (i) that describe soda ash recovery processes that utilize alkali solutions from solution mining or from dissolution of mined trona ore and (ii) that have a unit operation or step that involves conversion of bicarbonate to carbonate, e.g., via steam stripping, include U.S. Pat. No. 6,228,335 of Copenhafer et al. issued May 8, 2001; U.S. Pat. No. 6,576,206 of Copenhafer et al. issued Jun. 10, 2003; U.S. Pat. No. 6,589,497 of Smith issued Jul. 8, 2003; U.S. Pat. No. 7,645,435 of Braman et al. issued Jan. 12, 2010; and U.S. Patent Application Publication No. 2010/0066153 of Day et al. published Mar. 18, 2010.
Techniques for the removal of hydrogen sulfide from gas streams are described in many patents and in the technical literature. One of those techniques is the use of alkali carbonate or bicarbonate to remove hydrogen sulfide from gas streams containing both hydrogen sulfide and carbon dioxide, which is mentioned in the following patents.
U.S. Pat. No. 3,932,583 of Schievelbein issued Jan. 13, 1976 and U.S. Pat. No. 3,934,012 of Schievelbein issued Jan. 20, 1976 describe the preferential removal of hydrogen sulfide from a gas stream containing both hydrogen sulfide and carbon dioxide using an aqueous sodium bicarbonate solution.
U.S. Pat. No. 4,258,019 of Hiller et al. issued Mar. 24, 1981 describes a process for the selective removal at superatmospheric pressures of hydrogen sulfide from a gas stream containing both hydrogen sulfide and carbon dioxide using an aqueous alkali carbonate solution.
The present invention provides a method for removing hydrogen sulfide from a CO2-containing gas stream from a soda ash production facility in a gas-liquid absorber apparatus by utilizing absorber alkali solution streams that are readily available in existing soda ash production facilities and that, unlike prior art procedures, do not require regeneration.